Distinct anatomical visual pathways can be traced through the human central nervous system. These have been linked to specialized functions, such as encoding information about spatial forms (like the human face and text) and stimulus dynamics (flicker or movement). Our experiments are inconsistent with this strict division. They show that mechanisms responsive to flicker can alter form perception, with vision transiently sharpened by weakening the influence of flicker-sensitive mechanisms by prolonged exposure to flicker. So, next time you are trying to read fine print, you might be well advised to first view a flickering stimulus!Abstract
Human vision is surprisingly malleable. A static stimulus can seem to move after prolonged exposure to movement (the motion aftereffect), and exposure to tilted lines can make vertical lines seem oppositely tilted (the tilt aftereffect). The paradigm used to induce such distortions (adaptation) can provide powerful insights into the computations underlying human visual experience. Previously spatial form and stimulus dynamics were thought to be encoded independently, but here we show that adaptation to stimulus dynamics can sharpen form perception. We find that fast flicker adaptation (FFAd) shifts the tuning of face perception to higher spatial frequencies, enhances the acuity of spatial vision—allowing people to localize inputs with greater precision and to read finer scaled text, and it selectively reduces sensitivity to coarse-scale form signals. These findings are consistent with two interrelated influences: FFAd reduces the responsiveness of magnocellular neurons (which are important for encoding dynamics, but can have poor spatial resolution), and magnocellular responses contribute coarse spatial scale information when the visual system synthesizes form signals. Consequently, when magnocellular responses are mitigated via FFAd, human form perception is transiently sharpened because “blur” signals are mitigated.
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